在Java中給圖片加水印可以通過以下步驟實現：

1. 導入相關的類庫：

import java.awt.AlphaComposite;
import java.awt.Font;
import java.awt.Graphics2D;
import java.awt.RenderingHints;
import java.awt.image.BufferedImage;
import java.io.File;
import javax.imageio.ImageIO;

2. 加載原始圖片和水印圖片：

BufferedImage originalImage = ImageIO.read(new File("original.jpg"));
BufferedImage watermarkImage = ImageIO.read(new File("watermark.png"));

3. 創建一個新的圖片對象，大小與原始圖片相同：

BufferedImage combinedImage = new BufferedImage(originalImage.getWidth(), originalImage.getHeight(), BufferedImage.TYPE_INT_RGB);

4. 創建一個圖形對象并設置抗鋸齒和渲染模式：

Graphics2D graphics = (Graphics2D) combinedImage.getGraphics();
graphics.setRenderingHint(RenderingHints.KEY_ANTIALIASING, RenderingHints.VALUE_ANTIALIAS_ON);
graphics.setRenderingHint(RenderingHints.KEY_RENDERING, RenderingHints.VALUE_RENDER_QUALITY);

5. 將原始圖片繪制到新的圖片對象上：

graphics.drawImage(originalImage, 0, 0, null);

6. 設置水印圖片的透明度和位置，并繪制到新的圖片對象上：

AlphaComposite alphaComposite = AlphaComposite.getInstance(AlphaComposite.SRC_OVER, 0.5f); // 設置透明度為0.5
graphics.setComposite(alphaComposite);

int x = combinedImage.getWidth() - watermarkImage.getWidth() - 10; // 水印圖片的x坐標
int y = combinedImage.getHeight() - watermarkImage.getHeight() - 10; // 水印圖片的y坐標
graphics.drawImage(watermarkImage, x, y, null);

7. 添加水印文本：

graphics.setFont(new Font("Arial", Font.BOLD, 12));
graphics.drawString("watermark", 10, 20);

8. 保存新的圖片對象為文件：

ImageIO.write(combinedImage, "JPEG", new File("result.jpg"));

完整示例代碼如下：

import java.awt.AlphaComposite;
import java.awt.Font;
import java.awt.Graphics2D;
import java.awt.RenderingHints;
import java.awt.image.BufferedImage;
import java.io.File;
import javax.imageio.ImageIO;

public class ImageWatermarkExample {
    public static void main(String[] args) {
        try {
            BufferedImage originalImage = ImageIO.read(new File("original.jpg"));
            BufferedImage watermarkImage = ImageIO.read(new File("watermark.png"));

            BufferedImage combinedImage = new BufferedImage(originalImage.getWidth(), originalImage.getHeight(), BufferedImage.TYPE_INT_RGB);

            Graphics2D graphics = (Graphics2D) combinedImage.getGraphics();
            graphics.setRenderingHint(RenderingHints.KEY_ANTIALIASING, RenderingHints.VALUE_ANTIALIAS_ON);
            graphics.setRenderingHint(RenderingHints.KEY_RENDERING, RenderingHints.VALUE_RENDER_QUALITY);

            graphics.drawImage(originalImage, 0, 0, null);

            AlphaComposite alphaComposite = AlphaComposite.getInstance(AlphaComposite.SRC_OVER, 0.5f);
            graphics.setComposite(alphaComposite);

            int x = combinedImage.getWidth() - watermarkImage.getWidth() - 10;
            int y = combinedImage.getHeight() - watermarkImage.getHeight() - 10;
            graphics.drawImage(watermarkImage, x, y, null);

            graphics.setFont(new Font("Arial", Font.BOLD, 12));
            graphics.drawString("watermark", 10, 20);

            ImageIO.write(combinedImage, "JPEG", new File("result.jpg"));
        } catch (Exception e) {
            e.printStackTrace();
        }
    }
}

以上代碼將原始圖片和水印圖片合并為一張新的圖片，保存為result.jpg。其中，original.jpg是原始圖片，watermark.png是水印圖片，"watermark"是水印文本。您可以根據實際情況修改文件路徑和水印位置、透明度等參數。